Subsurface information obtained during a recent investigation has been combined with information from previous investigations to establish a stratigraphic framework for the Cretaceous rocks of southwestern Minnesota. This new framework makes possible the reconstruction of Late Cretaceous (Cenomanian to Campanian) paleogeography.

In southwestern Minnesota, the Greenhorn cyclothem is partly represented by an eastward transition from mostly marine strata of shelf and nearshore environments to mostly nonmarine strata of fluvial and deltaic depositional environments. Rocks of subsequent depositional cycles are extensively eroded and display less variation in depositional environment. The Upper Cretaceous sequence has been mapped as nine lithostratigraphic units. The combination of paleontologic information from past investigations and the stratigraphic framework yields a depositional history that begins with marine incursion in the late Cenomanian, continues with peak transgression in the early Turonian and regression through the remainder of the Turonian, and ends with erosion, creating an unconformity. A unit of Coniacian age overlies this erosional surface and represents another transgressive phase; although not evident, this unit may contain a lithologically similar regressive phase. An erosional unconformity forms the upper boundary of this unit. Finally, a poorly documented, mostly eroded unit similar to the Pierre Shale of eastern South Dakota occurs as small remnants, indicating a transgression in the Campanian. Distinctive clay-mineral assemblages that correspond to stratigraphic positions are further evidence of the postulated sea-level history.